A number of offset printing plate precursors for directly producing printing plates have hitherto been proposed, some of which have already been put into practical use. The most widely employed precursor is a photoreceptor having a photoconductive layer comprising photoconductive particles, such as zinc oxide, and a resin binder provided on a conductive support. A highly lipophilic toner image is subsequently formed on the photoconductive layer surface by an electrophotographic process. The toner image formed on the surface of the photoreceptor is then treated with an oil-desensitizing solution, called an etching solution, to selectively render the non-image areas hydrophilic thus producing an offset printing plate.
In order to obtain satisfactory prints, an offset printing plate precursor or photoreceptor must faithfully reproduce an original on the surface thereof; the photoreceptor surface should have a high affinity for an oil-desensitizing solution so as to render non-image areas sufficiently hydrophilic and, at the same time, should be water resistant. When used as printing plate, the photoconductive layer having a toner image formed thereon should adhere during printing, and should be receptive to dampening water so that the non-image areas can remain sufficiently hydrophilic to be free from stains, even after a large number of prints have been reproduced from the plate.
These properties are effected by the proportion of zinc oxide to resin binder in the photoconductive layer. Specifically, when the proportion of zinc oxide particles to resin binder in the photoconductive layer is decreased, the oil-desensitivity of the photoconductive layer surface is enhanced and background staining is decreased. However, the internal cohesive force and mechanical strength of the photoconductive layer itself is lowered resulting in the deterioration of the printing impression. On the contrary, when the proportion of a resin binder is increased, the background stain is increased although the printing impression is heightened. Background staining is related to the oil-desensitivity of the photoconductive layer surface. Not only does the ratio of zinc oxide to resin binder in the photoconductive layer influence the oil-desensitivity, but the oil-desensitivity also depends on the type of the resin binder employed.
Resins for use in photoreceptors include silicone resins as disclosed in JP-B-34-6670 (the term "JP-B" as used herein means an "examined Japanese patent publication"), styrene-butadiene resins as disclosed in JP-B-35-1950, alkyd resins, maleic acid resins and polyamides as disclosed in JP-B-35-11219, vinyl acetate resins as disclosed in JP-B-41-2425, vinyl acetate copolymers as disclosed in JP-B-41-2426, acryl resins as disclosed in JP-B-35-11216, acrylic acid ester copolymers as disclosed in JP-B-35-11219, JP-B-36-8510, JP-B-41-13946, etc. However, electrophotographic photoreceptors employing these resins each have various problems including (1) low chargeability of the photoconductive layer, (2) poor image reproducibility (in particular, dot reproducibility and resolving power), (3) low photoreceptivity, (4) insufficient oil-desensitivity of the photoconductive layer surface resulting in generation of background stains on the prints when offset printing is performed, even when subjected to an oil-desensitizing treatment for producing an offset master, (5) insufficient film strength of the photoconductive layer, resulting in loss of adhesion upon offset printing and fewer prints, and (6) sensitivity of the image quality to the environment at the time of image reproduction (e.g., high temperature and high humidity condition).
With respect to the offset master, the background stain resulting from insufficiency in oil-desensitization is a particularly serious problem. For the purpose of solving this problem, various binder resins in combination with zinc oxide have been developed for the prospect of enhancing the oil-desensitivity. Resins which enhance oil-desensitivity of the photoconductive layer include those discussed as follows: JP B-50-31011 discloses a resin having a molecular weight of from 1.8.times.10.sup.4 to 1.0.times.10.sup.5 and a glass transition point (Tg) of from 10.degree. C. to 80.degree. C., and which is prepared by copolymerizing a (meth)acrylate monomer and another monomer in the presence of fumaric acid, with a copolymer prepared from a (meth)acrylate monomer and a monomer other than fumaric acid; JP-A-53-54027 (the term "JP-A" as used herein means an "unexamined published patent application") discloses a ternary copolymer comprising a (meth)acrylic acid ester having a substituent which contains a carboxylic acid group apart from the ester linkage by at least 7 atoms; JP-A-54-20735 and JP-A-57-202544 disclose quaternary or quinary copolymers comprising acrylic acid and hydroxyethyl (meth)acrylate; and JP-A-58-68046 discloses a ternary copolymer comprising a (meth)acrylic acid ester having an alkyl group containing 6 to 12 carbon atoms as a substituent and a vinyl monomer containing a carboxylic acid group. However, even with the use of the above-described resins, which are said to enhance oil-desensitivity, the resulting offset masters are not sufficiently resistant to background stain, printing impression, etc. from a practical point of view.
On the other hand, resins of the type which contain functional groups capable of producing hydrophilic groups through decomposition have been examined as candidates for the resin binder. For example, the resins containing functional groups capable of producing hydroxyl groups by decomposition are disclosed in JP-A-62-195684, JP-A-62-210475 and JP-A-62-210476, and those containing functional groups capable of producing carboxyl groups through decomposition are disclosed in JP-A-62-21269.
These techniques disclose resins which produce hydrophilic groups by hydrolysis or hydrogenolysis in the presence of an oil-desensitizing solution or dampening water used during printing When used as resin binder for lithographic printing plate precursors, these resins are said to mitigate various problems including the aggravation of surface smoothness, the deterioration of electrophotographic characteristics, etc. These problems are thought to be caused by the strong interaction between the released hydrophilic groups and the surface of photoconductive zinc oxide particles when resins originally containing hydrophilic groups themselves are used as the resin binder. In addition, the affinity of the non-image part for water, rendered hydrophilic by an oil-desensitizing solution, is said to be further strengthened by the aforesaid hydrophilic groups produced by decomposition of the resins to make a clear distinction between the lipophilic image part and the hydrophilic non-image part. At the same time, these resins are said to prevent printing ink from adhering to the non-image part upon printing thereby enabling printing of a large number of clear prints free from background stains.
Even these resins, however, do not fully prevent background staining Satisfactory printing impression is also not fully realized. Specifically, when such hydrophilic group-producing resins are used in a large proportion to further improve the affinity of the non-image part for water, the durability of the resulting printing plate is impaired. The hydrophilic groups produced by decomposition render the non-image part soluble in water while increasing the affinity for water.
Accordingly, methods were needed for further enhancing the affinity of the non-image part for water while at the same time improving durability.